1. Field of the Invention
The present invention relates to an ATM (Asynchronous Transfer Mode) exchange, and in particular, to an ATM exchange performing flow control of traffic by a feedback method.
2. Description of the Related Art
In ATM exchanges according to an prior art technique, for example, as describe in a document (F. Bonomi and K. W. Fendick, "The Rate-Based Flow Control Framework for the Available Bit Rate ATM Service", IEEE Network, pp 25-39, March/April 1995) etc., a method is employed in which every time a queue length of cells within a buffer exceeds a certain constant threshold, or when it is predicted to exceed the threshold, the feedback is applied.
Furthermore, as a modified example of the feedback method, it is proposed to apply the feedback when a maximum queue length within a constant time interval exceeds a threshold a plurality of times successively as shown in a document "Naotoshi Watanabe, Toshio Soumiya, Masafumi Katoh, Akira Nakata: The Institute of Electronics, Information and communication Engineers Technical report: The Society for Switching System: SSE93-30 (1993-7)".
In an ATM exchange according to a prior art technique, as shown in FIG. 2, a congestion control method using a trigger is employed in which every time the queue of cells within a buffer provided in the ATM exchange exceeds a constant threshold, or it is predicted to exceed, the feedback is applied to control the inflow. In other words, in the prior art control method, whenever the queue of cells existing within the buffer exceeds a certain threshold, even when the queue will be resolved naturally, the feedback is applied to activate the congestion control.
Now, the statistical behavior of queue lengths of cells caused within the buffer of the exchange can be regarded as the Markov process wherein the queue lengths of cells are probability variables. That is, as shown in FIG. 3, according to the theory of the Markov process, in the Markov process where a steady distribution exists, when a certain value of the probability variables is observed at time t=o, assuming that the drift coefficient is "1", at the point of time after a certain fixed time, for example, t=1/128 is elapsed, queue lengths are distributed with probability as shown by a curve B. similarly, it is known that the probability distribution after t=1/2 is elapsed, coincides almost with a steady distribution (curve F : t=infinitive).
Hereinafter, in the present specification, a required time until a probability distribution almost coincides with the steady distribution in the Markov process is described as a time constant.
The fact that a queue length of cells exceeds a threshold corresponds to that a certain value of the probability variables has been observed. Also, when a queue length does not recover the threshold or lower until the time constant elapses, this correspond to that a queue length exceeding the threshold has been produced in the steady distribution.
The threshold is assumed to be a queue length which seldom occurs in the steady distribution under a normal load. In this case, when the time constant elapses after the queue length exceeds the threshold, as far as the load is a normal load, it means that in most cases the queue length will recover the threshold or lower. For example, assuming that the threshold is a queue length which occurs at the rate of 1/10.sup.4 in the steady distribution, it will recover the threshold or lower at the rate of 99.99%.
In the congestion judging method according to the prior art technique employing the threshold trigger method, it is impossible to judge whether the detected congestion is light congestion which recovers naturally, or heavy congestion which requires feedback control.
In ATM exchanges, under a normal load, the light congestion caused by collision of bursts by chance occurs overwhelmingly, and in most cases the congestion recovers naturally.
Therefore, in the method according to the above-mentioned prior art technique wherein the feedback is applied regularly, since the feedback control is performed every time even to the light congestion which recovers naturally, a problem is involved in which the overhead is increased due to the traffic restriction, and the utilization rate of outgoing lines is lowered.
Furthermore, the control method described in the above-mentioned document, the switching system study society: SSE93-30(1993-7) which is effective in the case of statistically multiplexing random traffic is applied to an ATM exchange for statistically multiplexing burst traffic. In this control method, it is impossible to judge whether, as shown in FIG. 1A, the queue length exceeds a threshold intermittently in each observation period due to light congestion which recovers naturally, or as shown in FIG. 1B, heavy congestion is occurring wherein the queue length exceeds the threshold continually over the observation periods.
Accordingly, also in this control method, since the feedback control is performed eventually even against the light congestion which recovers naturally, the problem of the increase of the overhead due to the traffic restriction, and the lowering of the utilization rate of outgoing lines cannot be resolved.
On the other hand, the standardization of an ABR (Available Bit Rate) service is studied in the international standardization agency such as the ITU (International Telecommunication Union), etc. In the ABR service, the amount of traffic is adaptively controlled according to the seriousness of congestion of queue lengths of cells.
In order to realize this service, an ATM exchange is required which performs adaptive traffic control by feedback controlling the terminals connected thereto in accordance with the seriousness of congestion, and controlling the amount of traffic adaptively.
In order to perform the adaptive traffic control, it is indispensable to provide a function to judge the seriousness of congestion, and to determine the amount of feedback according to the seriousness of congestion. In particular, what is fatal to the ATM exchange is that, as shown in FIG. 1B, the occurrence of queue lengths exceeding the threshold continues.
Even if the queue length exceeds the threshold instantaneously, if the queue length recovers a value equal to or lower than the threshold within a sufficiently short period, it will be possible to reduce the cell loss probability to permissible cell loss probability or lower without performing the feedback control.
Accordingly, in order to realize the ABR service, it is necessary as a minimum requirement to judge whether the occurred congestion of queues is light congestion which recover naturally, or heavy congestion which requires feedback control.